Drums, barrels and other containers are widely used to store, transport and dispense chemicals and industrial fluids. An example of such a drum is disclosed in U.S. Pat. No. 6,045,000, which is owned by the owners of the present invention and is hereby fully incorporated herein by reference. In some industries, such as semiconductor processing, the liquids contained may be highly volatile and may evolve vapors or gases that will build pressure within the container unless vented. An example of such a fluid is hydrogen peroxide, which will evolve oxygen. Also, like hydrogen peroxide, the fluids may be toxic, flammable or otherwise hazardous. Thus, it is important that the fluid be contained within the drum and not allowed to escape. In addition, the contained fluids must often be maintained in an extremely pure condition, and any outside contaminants must be prevented from entering the container through vents or other openings. An example of a containment system and dispense head incorporating many of these features is disclosed in U.S. Pat. No. 6,079,597, also hereby fully incorporated herein by reference.
Drums and closure devices, including vents, used for shipping hazardous chemicals, such as many of the chemicals used in semiconductor processing, must pass rigorous tests required by the U.S. Department of Transportation for transport within the United States and the United Nations for transport internationally. One of these tests, required by 49 C.F.R. § 178.603 (2001), requires that the drum be inverted dropped. The drum must maintain its structural integrity and no part must leak fluid after the test.
During the drop test described above, a venting device can experience a sharp pressure reversal. When the drum first makes impact with the ground, the drum deflects, compressing the fluid inside and exerting a liquid pressure on the vent from inside the drum. Next, however, when the drum may resiliently spring back and the liquid moves back away from the vent, air will be drawn through the vent in the opposite direction.
Various devices have been developed for venting drums and other containers so as to allow evolved vapor and gases to escape while preventing the escape of liquid and the entry of contaminants. One such prior device includes a threaded plastic plug portion with one or more apertures in the center of the plug. A membrane is affixed over the apertures and is fastened to the plug at the margins. The plug is threaded into a corresponding threaded opening in the top of the drum with the membrane facing inward into the drum. The membrane is generally a piece of PTFE material on a backing scrim material. The membrane and scrim has a thickness of from about 0.015 to 0.020 inch. The PTFE membrane allows gas and vapor molecules to escape through the apertures and through the pores of the membrane, while preventing the escape of liquid.
A problem with these prior devices is that, unless the membrane and scrim assembly is made relatively thick, the inrush of air through the vent occurring during the drop test as described above tends to rupture the membrane or tear it loose from the plug portion. In addition, the thick membrane material restricts flow through the vent, leading to diminished vent performance. The thick material can become clogged with dried chemicals, leading to eventual failure of the vent. Another problem is that the membrane is open to contact from foreign objects and may be easily damaged as a result.
Other prior art vents have been developed wherein protective structures are placed proximate the membrane so as to protect the membrane from contact. In these vents, however, chemicals can be retained in the protecting structure if the drum is not stored in an upright condition, and may coagulate or dry adjacent to the membrane. This leads to eventual failure of the venting device as described above.
Thinner membranes have been used in some prior art vents to improve venting effectiveness. These membranes can be as thin as 0.002 inch and may have pore sizes on the order of 0.2 microns. A protector plate structure is positioned on the inner side of the vent over, and slightly spaced apart from, the membrane. The protector plate serves two functions in this device. First, it provides protection from contact for the membrane, which is subject to damage from even light contact with any hard object due to its thinness. Secondly, it serves to restrain the membrane during the air inrush phase of drop testing, thereby preventing the membrane from rupturing. The protector plate may generally placed no more than about 0.030 inch away from the membrane without incurring a significant risk of rupture during drop testing.
A problem with the thinner membrane vents with protector plates, however, is that chemicals can “hang-up” in the protector plate structure and may accumulate around the membrane. These chemicals may coagulate or dry, leading to failure of the vent. Also, the relatively solid structure of the protector place, necessary to adequately protect and restrain the membrane, may result in a loss of venting capacity in some cases.
What is needed in the industry is a more effective and more durable venting device for a drum.